CN104569776B - A kind of method for measuring electronics effective mobility in the OLED for having multilayer luminescent layer - Google Patents

Abstract

The invention belongs to the technical field of organic optoelectronics, in particular to a method for measuring the effective mobility of electrons in an OLED device containing a light-emitting layer. The test mechanism of the present invention is that in OLEDs, the injection barriers and mobility of electrons and holes are different. Square wave pulse currents with different pulse widths are used to drive OLEDs. By analyzing the current waveforms and emission spectra of OLEDs, it can be concluded that the electrons are effective. The time required to pass through a certain light-emitting layer; if the thickness of the light-emitting layer and the electric field strength in the OLED are known, the effective mobility of electrons can be obtained according to the calculation formula.

Description

A Method for Measuring the Effective Mobility of Electrons in OLED Devices with Multiple Emissive Layers

technical field

The invention belongs to the technical field of organic optoelectronics, and in particular relates to a method for measuring the effective mobility of electrons in an OLED device containing a light-emitting layer.

Background technique

OLED is a new generation of organic light-emitting devices, which are currently widely used in display fields such as mobile phones and televisions. With the continuous development of technology, white light OLED has begun to enter people's field of vision, and is expected to become a light source for general lighting in the future.

OLED is a DC-driven device. During operation, electrons and holes are injected into the OLED from the cathode and anode respectively, and then reach the light-emitting layer after passing through the carrier transport layer to realize composite light emission. The photoelectric properties of OLEDs are closely related to the injection and transport of carriers. Due to the different materials used in different layers, the injection and transport of electrons and holes are also different. Moreover, due to the inevitable existence of some defects and non-radiative recombination centers in the OLED device, the carrier transport and recombination process will inevitably be affected. Especially when the device processing technology is improper, or the device is aged for a long time, the above-mentioned defects and non-radiative recombination centers will increase significantly, which will significantly affect the transport of carriers, thereby affecting the performance of OLEDs. Therefore, it is very important to measure the transport of carriers in OLEDs to judge the processing quality and final performance of OLED devices, and to find out the factors that restrict the performance improvement of OLEDs.

At present, the methods for measuring carrier mobility in organic optoelectronic devices mainly include:

1. Time-of-flight method

The material to be tested is made into a sample with a certain thickness, placed between two electrodes, and one end of the electrode is irradiated with a transient light to generate carriers, which are transported under the action of an external electric field. Based on the sample thickness, the time it takes for carriers to reach the other end of the electrode, the mobility can be calculated.

2. Space-limited current method

The method measures the steady-state space-limited current under the condition of no trap, and then infers the carrier mobility according to the dark current j-voltage U curve of the organic material.

3. Injection-type instantaneous dark current method

The method is to infer the carrier mobility by injecting carriers and measuring the transient behavior of the space charge-limited current of the carriers in the dark state.

4. Transient electroluminescence method

This method is similar to the injection-type transient dark current method, and the transient electroluminescence also uses a pulse wave to generate a transient voltage. However, this method collects transient luminescence signals, not current signals. The organic electroluminescent device emits light under the drive of transient voltage, and uses a high-response silicon-based photomultiplier tube to detect the light emitted by the device and store it in an oscilloscope. The carrier mobility can be calculated from the detected transient electroluminescence time.

It is not difficult to see that the above test method is mainly aimed at measuring the carrier transport of a single material under ideal conditions; OLED is a device composed of multiple layers of organic materials, and the materials and thicknesses used in different layers are different. Not the same, the defects inside the layer are also complicated, therefore, these methods are not suitable for OLED devices. In this invention, we will provide a method that can directly measure the effective mobility of electrons in OLED devices.

Contents of the invention

The purpose of the present invention is to provide a method for directly measuring the effective mobility of electrons in OLED devices containing multiple (ie, two or more) light-emitting layers, which can objectively reflect the defects in OLED devices.

The method for the effective mobility of electrons in an OLED device whose measurement layer contains a multilayer light-emitting layer provided by the present invention, the specific steps are as follows:

Step 1: Put the OLED to be tested into the test cavity;

Step 2: Use the driving power to supply power to the OLED, and use the electrical parameter acquisition equipment, optical parameter acquisition equipment, and thermal parameter acquisition equipment to collect the optical characteristics, electrical characteristics, and thermal characteristics of the OLED respectively; including: the electrical parameter acquisition equipment collects the voltage when the OLED is working The effective value of V, the optical parameter acquisition equipment collects the spectrum emitted by the OLED, and records the pulse width of the square wave pulse current corresponding to the appearance of different spectra, and calculates the effective time for electrons to pass through a certain light-emitting layer inside the OLED. Through the time τ; the thermal parameter acquisition device collects the working temperature of the OLED;

Step 3: According to the electrical parameter and optical parameter data collected above, and the OLED structure data, the effective mobility of electrons is calculated according to the formula.

Wherein, the structure of the OLED includes a cathode, a carrier layer, a light emitting layer and an anode; the carrier layer includes an electron injection layer, an electron transfer layer, a hole blocking layer, a hole injection layer, a hole transfer layer layer, electron blocking layer or all of them; the number of layers of the light-emitting layer can be two or more.

Further, the test cavity described in step 1 may be a light flux sphere, or other closed cavity whose inner wall is painted white or black.

Further, the driving power described in step 2 is a constant current source, which can output a square wave pulse current with a pulse width of 0-10 μs and an amplitude of 0-500 mA. The electrical parameter acquisition equipment includes an oscilloscope, a voltage probe, and a current probe. The optical parameter acquisition device is a spectrometer, and the photosensitive device used in it is a CCD or ICCD or a photomultiplier tube, which can measure the radiation of 200nm-900nm. The thermal parameter acquisition device is a thermocouple capable of measuring the temperature from -20°C to 100°C.

Further, in step 2, when the test starts, the drive power supply outputs a square wave pulse current, the amplitude of which is the rated current value of the OLED to be tested, and the initial value of the pulse width can be set to 0-10ns; after the test starts, keep the The amplitude of the square wave pulse current is kept constant, and the pulse width is gradually increased. In OLEDs, the injection barrier of holes is smaller than that of electrons, and the mobility of holes is greater than that of electrons. Therefore, at the beginning of the test, a large number of holes are rapidly injected into the OLED, while electrons are injected in a small amount and at a slow rate. At this time, electrons either cannot reach the light-emitting layer of the OLED, and can only recombine with holes, defects, and non-radiative recombination centers in the carrier layer; or electrons can reach the junction of the carrier layer and the light-emitting layer, but only first consistent with defects and nonradiative recombination centers at this interface. In the above two cases, the electrical parameter acquisition device can detect that there is a pulse current flowing in the OLED, and obtain the effective value V of the voltage when the OLED is working. But the optical parameter collection system cannot collect any spectrum. As the pulse width of the square wave pulse current increases, more and more electrons are injected, and the depth of electron injection in the OLED device increases, and the recombination with defects and non-radiative recombination centers gradually reaches saturation, and some electrons begin to reach the OLED. The interior of the light-emitting layer recombines with holes to emit light. At this time, the optical parameter collection system can collect the spectrum emitted by the OLED. If there are two luminescent layers, as the pulse width of the square wave pulse current increases, electrons first arrive at the luminescent layer closest to the cathode, and then arrive at the second closest luminescent layer from the cathode; at this time, the optical parameter acquisition system The corresponding spectra will be collected sequentially. The case where the light-emitting layer is multi-layer can be deduced by analogy. Record the pulse width of the square wave pulse current corresponding to the appearance of different spectra, and based on this, the effective passage time for electrons to effectively pass through a certain light-emitting layer inside the OLED can be calculated.

Further, the specific calculation method of the effective passage time is:

(1) The light-emitting layer is two layers (light-emitting layer 1, light-emitting layer 2)

The optical parameter collection device is just able to detect the spectrum emitted by the luminescent layer 1, and the pulse width of the square wave pulse current recorded at this time is t ₁ . Increase the pulse width until the optical parameter acquisition device can just detect the spectrum emitted by the light-emitting layer 2 , and record the pulse width of the square wave pulse current at this time as t ₂ . The effective transit time τ=t ₂ −t ₁ for electrons through the light-emitting layer 1 is then.

(2) The light-emitting layer is multi-layered (light-emitting layer 1, light-emitting layer 2...light-emitting layer n)

The optical parameter acquisition device can just detect the spectrum emitted by the luminescent layer n, and the pulse width of the square wave pulse current recorded at this time is t _n . Increase the pulse width until the optical parameter acquisition device can just detect the spectrum emitted by the light-emitting layer n+1, and record the pulse width of the square wave pulse current at this time as t _n+1 . Then the effective transit time τ=t _n+1 -t _{n for electrons to pass through the light-emitting layer n} .

Further, the thermocouple is used to detect the operating temperature of the OLED to be tested, and the change of the operating temperature should be within ±1°C during the test.

Further, the structural data of the OLED described in step 3 includes: the total thickness of the OLED , the thickness d of the light-emitting layer through which electrons pass; is the electron effective mobility The calculation formula is:

Wherein, τ is the effective transit time, and V is the effective value of the OLED working voltage collected by the electrical parameter collection device.

The test method of the present invention takes factors such as defects and non-radiative recombination into consideration in the OLED device, and can intuitively describe the effective migration of electrons in the OLED device.

Description of drawings

Figure 1 Schematic diagram of the test circuit.

Fig. 2 Schematic diagram of the OLED structure to be tested.

Fig. 3 Schematic diagram of OLED working current.

detailed description

The specific embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

As shown in FIG. 1 , the OLED 101 to be tested is put into a light flux sphere 102 , and finally put into a constant temperature box 103 as a whole. The OLED is driven by a programmable current source 104 , the working current and voltage are collected by an oscilloscope and a corresponding probe 105 , and the emission spectrum is collected by a spectrometer system 106 . The temperature of the OLED during operation is monitored by thermocouple 107 .

The schematic diagram of the structure of the OLED used in the experiment is shown in Fig. 2 . During operation, holes 203 and electrons 204 are respectively injected from the anode 201 and cathode 202 of the OLED, pass through the carrier transport layer 205 and reach the blue-green light-emitting layer 206 and the red-light emitting layer 207 to recombine and emit light. When the power supply 104 outputs a pulsed square wave current, the schematic diagram of the current flowing in the OLED is shown in FIG. 3 . Due to the characteristics of the organic material itself, the injection of the holes 203 is easier than the electrons 204, and the mobility of the holes 203 inside the OLED is much greater than that of the electrons 204. Therefore, immediately after the circuit is turned on, a large number of holes 203 are rapidly injected into the OLED device, while only a small amount of electrons 204 are injected. At this time, the recombination of electrons 204 and holes 203 does not occur in the light-emitting layers 206, 207, but in the carrier transport layer 205 near the cathode side or in the carrier transport layer 205 and blue-green light emission. layer 206 interface. Moreover, the injected electrons may also recombine with various defects and non-radiative recombination centers inside the OLED. Therefore, the oscilloscope 105 can detect current flowing in the OLED, but the spectrometer system 106 cannot collect any spectrum. As the pulse width of the pulsed square wave current is continuously increased, more and more electrons are injected, and the injection depth becomes deeper and deeper, all kinds of defects and non-radiative recombination centers are saturated, and the electrons 204 and holes 203 are finally Recombined in the blue-green light-emitting layer 206, the OLED emits blue-green light. This corresponds to the part of the current curve from the origin to segment 3A in FIG. 3 . When the pulse width of the pulse square wave current continues to increase, the number of electrons injected into the blue-green light-emitting layer 206 increases, and the intensity of the blue-green light emitted by the OLED increases; moreover, the injection depth of electrons further increases, and electrons begin to enter the red light. In the light-emitting layer 207, the OLED begins to emit red spectrum. This corresponds to the curves of segments 3A-3B in FIG. 3 . Therefore, the pulse width corresponding to sections 3A-3B is actually the time τ for electrons to effectively pass through the blue-green light-emitting layer 206 . Assuming that the thickness of the blue-green light-emitting layer is d, the total thickness of the OLED is d _total , and the voltage across the OLED is V, then the effective mobility μ _e of electrons in the blue-green light-emitting layer can be calculated by the following formula:

.

Claims (6)

1. a kind of measurement layer contains the method for electronics effective mobility in the OLED of multilayer luminescent layer, comprise the following steps that：

Step 1：OLED to be measured is put into test chamber；

Step 2：OLED power supplies are given with driving power, power utilization parameters collection equipment collection OLED electrical characteristics, with light parameter acquisition Equipment collection OLED light characteristic, the thermal characteristics with thermal parameter collecting device collection OLED；Including：Electrical parameter collecting device gathers The virtual value V of voltage when being worked to OLED；Optical parameter collecting device collects the spectrum that OLED is sent, and records and do not share the same light The pulsewidth of current corresponding square wave pulsed current is composed, calculates electronics accordingly effectively by a certain luminescent layer inside OLED Effective passage time τ；Thermal parameter collecting device collects OLED operating temperature；

Step 3：According to the above-mentioned electrical parameter collected and optical parameter data, and OLED structure data：OLED gross thickness, The thickness d for the luminescent layer that electron institute passes through, the effective mobility of electronics is calculated according to formula：

。

2. according to the method for claim 1, it is characterised in that：Described OLED, its structure include negative electrode, carrier layer, Luminescent layer and anode；The carrier layer includes electron injecting layer, electron transfer layer, hole blocking layer, hole injection layer, hole It is several or whole in migrating layer, electronic barrier layer.

3. according to the method for claim 1, it is characterised in that：Test chamber described in step 1 is that light leads to ball, or its He whitewashes inwall or the airtight cavity of blacking.

4. according to the method for claim 1, it is characterised in that：Driving power described in step 2 is constant current supply, defeated Go out the square wave pulsed current that pulsewidth is 0-10ms, amplitude is 0-500mA；The electrical parameter collecting device includes oscillograph, voltage Probe and current probe；The optical parameter collecting device is spectrometer, and its sensor devices used is CCD or ICCD or photomultiplier transit Pipe, can measure 200nm -900nm radiation；The thermal parameter collecting device is thermocouple, can measure -20 DEG C -100 DEG C Temperature.

5. according to the method for claim 1, it is characterised in that：In step 2, when test starts, the driving power output Square wave pulsed current, its amplitude are OLED to be measured load current value, and pulsewidth initial value is set to 0-10ns；After test starts, protect It is constant to hold the amplitude of the square wave pulsed current, gradually increases pulsewidth, at this moment, electrical parameter collecting device can be detected in OLED There is pulse current to flow through, and obtain the virtual value V of voltage during OLED work；

With the pulsewidth increase of the square wave pulsed current, injected electrons quantity is more and more, and electronics is noted in OLED The depth increase entered, saturation is progressivelyed reach with defect and the compound of non-radiative recombination center, and some electronics start to reach OLED's Lighted inside luminescent layer with hole-recombination, at this moment, optical parameter acquisition system can collect the spectrum that OLED is sent：For hair Photosphere is two layers of situation, and with the increase of the pulsewidth of the square wave pulsed current, electronics first reaches the hair nearest apart from negative electrode Photosphere, the luminescent layer near apart from negative electrode second is then reached again, now, optical parameter collecting device collects corresponding light successively Spectrum；In the case of luminescent layer is multilayer, by that analogy；Record square wave pulsed current corresponding when different spectrum occur Pulsewidth, calculate electronics accordingly is effectively by effective passage time of a certain luminescent layer inside OLED, circular：

In the case of luminescent layer is two layers：

Described optical parameter collecting device can detect the spectrum that the first luminescent layer is sent just, then record square wave now The pulsewidth of pulse current is t₁；Increase pulsewidth, until the optical parameter collecting device can detect the second luminescent layer institute just The spectrum sent, the pulsewidth for recording square wave pulsed current now are t₂, then effective passage time that electronics passes through luminescent layer 1 τ=t₂-t₁；

In the case of luminescent layer is multilayer：

Described optical parameter collecting device can detect the spectrum that luminescent layer n is sent just, then record square wave arteries and veins now The pulsewidth for rushing electric current is t_n, increase pulsewidth, sent until the optical parameter collecting device can detect luminescent layer n+1 just Spectrum, the pulsewidth for recording square wave pulsed current now is t_n+1, then effective passage time τ that electronics passes through luminescent layer n =t_n+1-t_n。

6. according to the method for claim 1, it is characterised in that：The thermal parameter collecting device collects OLED work temperature Degree, it changes within ± 1 DEG C.